MARIE CURIE'S CHOICE of a thesis topic was influenced by two recent discoveries
by other scientists. In December 1895, about six months after the Curies married,
German physicist Wilhelm Roentgen discovered a kind of ray that could travel
through solid wood or flesh and yield photographs of living people's bones.
Roentgen dubbed these mysterious rays X-rays, with X standing for unknown. In
recognition of his discovery, Roentgen in 1901 became the first Nobel laureate
in physics.

In early 1896, only a few of months after Roentgen's discovery, French physicist
Henri Becquerel reported to the French Academy of Sciences that uranium compounds,
even if they were kept in the dark, emitted rays that would fog a photographic
plate. He had come upon this discovery accidentally. Despite Becquerel's intriguing
finding, the scientific community continued to focus its attention on Roentgen's
X-rays, neglecting the much weaker Becquerel rays or uranium rays.

THE IGNORED URANIUM RAYS appealed to Marie Curie. Since she would not have
a long bibliography of published papers to read, she could begin experimental
work on them immediately. The director of the Paris Municipal School of Industrial
Physics and Chemistry, where Pierre was professor of physics, permitted her
to use a crowded, damp storeroom there as a lab.

A clever technique was her key to success. About 15 years earlier, Pierre and
his older brother, Jacques, had invented a new kind of electrometer, a device
for measuring extremely low electrical currents. Marie now put the Curie electrometer
to use in measuring the faint currents that can pass through air that has been
bombarded with uranium rays. The moist air in the storeroom tended to dissipate
the electric charge, but she managed to make reproducible measurements.

"Instead of making these bodies act upon
photographic plates, I preferred to determine the intensity of their radiation
by measuring the conductivity of the air exposed to the action of the rays."

With numerous experiments Marie confirmed Becquerel's observations that the
electrical effects of uranium rays are constant, regardless of whether the uranium
was solid or pulverized, pure or in a compound, wet or dry, or whether exposed
to light or heat. Likewise, her study of the rays emitted by different uranium
compounds validated Becquerel's conclusion that the minerals with a higher proportion
of uranium emitted the most intense rays. She went beyond Becquerel's work,
however, in forming a crucial hypothesis: the emission of rays by uranium compounds
could be an atomic property of the element uranium--something built into the
very structure of its atoms.

MARIE'S SIMPLE HYPOTHESIS would prove revolutionary. It would ultimately contribute
to a fundamental shift in scientific understanding. At the time scientists regarded
the atom--a word meaning undivided or indivisible -- as the most elementary
particle. A hint that this ancient idea was false came from the discovery of
the electron by other scientists around this same time. But nobody grasped the
complex inner structure or the immense energy stored in atoms. Marie and Pierre
Curie themselves were not convinced that radioactive energy came from within
atoms--maybe, for example, the earth was bathed in cosmic rays, whose energy
certain atoms somehow caught and radiated? Marie's real achievement was to cut
through the complicated and obscure observations with a crystal-clear analysis
of the set of conclusions that, however unexpected, were logically possible.

Marie tested all the known elements in order to determine if other elements
or minerals would make air conduct electricity better, or if uranium alone could
do this. In this task she was assisted by a number of chemists who donated a
variety of mineral samples, including some containing very rare elements. In
April 1898 her research revealed that thorium compounds, like those of uranium,
emit Becquerel rays. Again the emission appeared to be an atomic property. To
describe the behavior of uranium and thorium she invented the word "radioactivity"
--based on the Latin word for ray.